Fuel priming / air leak theory
 

hi all,

I replaced my diesel fuel filters on my 300D yesterday.

Replaced, filled with fuel, primed, started right up. Sporadically now on startup or seconds thereafter it has a couple stutters which are air in the fuel system.

Is it possible I just need have trapped air somehow and need to re-prime, or do I definitely have an air leak?

I think the answer is air leak, but just wanted to confirm.

thanks,
dd

Did you fill the secondary filter with diesel before you started the priming process?

whunter - I will give you a call when I am on break. thank you!

hey army!

I did.

My system is a tad 'special' due to my WVO conversion (that has been in place for 3 yrs / 40k+). The lift pump is plumbed after the secondary filter (as opposed to before it), so that assembly is under vacuum as opposed to pressure.

My priming procedure involves loosening the bolt at the inlet to the IP since that the easiest place for me to access that's under pressure. I then pump until I feel that I have gotten the air out (and spilled diesel all over the ground) and then close the bolt and pump until I hear the 'buzz' sound from the IP indicating prime.

My question really revolves around if the engine goes on a 50 mile trip just fine, and then has air upon restart, does that definitively indicate an air leak?

Most definitely!

Do you have a vacuum gauge on the common of your supply valve? That is the most useful diagnostic tool for running WVO.

Do you have a clear return line? That is another most useful diagnostic tool.

Did you ever fix your starter problems?

Yes, the wire going to the starter was loose :o:o:o:o:o:o:o I should go back and update that thread.

I don't have a vac gauge or clear tubing. The car stutters when going back to diesel from VO so I know it's a diesel side problem.

I did go down to the garage and see that my diesel side clear pre filter half empty. That pretty much nails it....leakity leakity. I tightened hose clamps and the secondary filter.

Those Mityvac things look great but they cost a fortune here. I just bought a vacuum gauge for 12 euros and a few connection pieces with a bit of tubing. It is better than nothing. (I'm keeping an eye out for a Mityvac thing though - when the price is right I will pounce!)

Oh yeah I agree with funola about a leak somewhere (if that helps)...

And that is exactly where your problem is! You can't do that without modifying the filter mounting bracket. You are drawing air into the system from the fuel return line, through the secondary filter bleed hole. You need to block it.

It must have been modified since he ran 40k with no problems. Wouldn't hurt to check the modification though.

The return banjo bolt is modified to be 'solid' instead of hollow.

I tightened all the clamps and didn't see any fuel level drop overnight in the prefilter, but still had some stuttering upon switching back to diesel when I arrived at work.

Next chance I get I'll re-lube and re-seat the secondary diesel filter and secondary filter banjo bolt. I didnt touch anything in the fuel system besides the immediate connections to the filters.

Thank you all for the help thus far.

How much do they cost there? If you can get an old home window AC unit, fridge etc, the compressor makes an excellent vac pump. Not portable like a Mityvac but gets the job done.

In the UK they are more or less the same as in the US when exchange rate etc is taken into account - however, lots of people find it traumatic posting stuff out of one country to another! I'm off back visiting the family in the summer so there's a chance to pick up stuff for myself then. As for prices here and in Germany - well wow! I won't even post them up 'cos they'll cause an outrage...

Thanks for the compressor tip - I might be able to get the same effect with a vacuum cleaner - the motor might race a bit though!

I was going to start low-tech with a bicycle pump though...

FYI
 

Thank you - that's another one book marked!

FYI
 

I will try to get pictures of my small units for you.

.

Curious if you ever got this resolved. It is very difficult to purge air out of a modified Mercedes fuel system.

Do you have this fuel configuration:

tank > pre filter > filter > 3 way valve > priming pump > IP ?

It is just about impossible to purge air out of the system using the priming pump in this configuration with the FSM method. A Mityvac on the return won't work either since the banjo bolt is plugged and you are pulling the pressure relief valve closed.